This invention relates to an error detecting device for an optical head which detects a focussing error and a tracking error.
Recently, products utilizing the laser technique have been developed together with the progress of the semiconductor laser. Presently, among the products, especially the optical disc has attracted attention. Among the presently commercially available optical discs, read only optical discs, such as compact discs (CD) and laser discs (LD), and write once optical discs for use in document files, video files, memories of computers, etc. have entered practicable stages. In addition, erasable optical disks which can write, read and erase information will be soon commercialized.
The optical head is an essential member for writing and reading information in and from optical discs. In order to write and read information by an optical head it is necessary to control the optical head so that a beam of laser light can correctly irradiate a desired track of an information recording medium.
The error detecting device for the optical head detects a focussing error and a tracking error of a beam of laser light irradiating an information recording medium. Based on detecting signals of the focussing error and the tracking error, the optical head is feedback controlled.
In the error detecting device for an optical head, a focussing error and a tracking error are detected, based on a reflected light of a beam of laser light irradiating an optical disc. The reflected light for the error detection is formed by a convex lens and a cylindrical lens into an astigmatic pencil of rays, which has astigmatic wave fronts. This astigmatic pencil of rays is focussed on a quadrant photo detector. Based on the light detecting signals from respective quarter sections of the quadrant photo detector, a focussing error signal is produced.
As described above, in the conventional error detecting device, a convex lens and a cylindrical lens are used to form a focussing error detecting light into an astigmatic pencil of rays. A consequent problem is that the miniaturization of the error detecting device is difficult.
Furthermore, the curvature of radius of the cylindrical lens has to be large in order to improve the error detecting precision. Increasing the curvature of radius of the cylindrical lens results in small production tolerances in eccentricity of optical elements and generatrix of alignment of the convex cost, and the difficulty of alignment of the convex lens, the cylindrical lens and the quadrant photo detector.